FITTING A MATCH BARREL THE JACK FUSELIER WAY
First let's clear up a common misconception about the
space between the hood and the breech.
#1 The clearance between the hood end and breech is
irrelevant, as long as it is enough to function with fouling present.
#2 For tight grouping the barrel should be fitted so that
the hood goes into the breech slot precisely with minimum clearance.
#3 For the hood to be fitted this precisely, the underlug
must first be fitted into its slot so it moves without side play.
This guides the hood into the breech.
#4 As the barrel goes into battery, it is guided by the
underlug and the hood must be fitted to the breech slot to follow this
movement. The hood is trimmed in width on each side whatever amount is
needed so that it will slide smoothly into lockup without any clashing
or side force from the sides of the breech slot. No more than necessary
for clearance should be taken from the sides of the hood. Excess
clearance contributes to poor grouping.
WHAT ACTUALLY HAPPENS WHEN THE GUN IS FIRED
#5 The barrel does not move backward while the slug is still
in it. It is driven forward and rotates in whatever slack is available
because of the thrust and torque of the slug being driven through the
rifling. The barrel must be held tightly both at the underlug and at the
breech for minimum movement.
#6 Fore and aft slack in the top lugs is not desirable on a
gun that is intended to fire high power loads. A gun to shoot "hot"
loads should have minimum F & A slack. While in battery before firing the
barrel is forced forward by the top lug engagement with the slide as
the slide is forced forward by the recoil spring. Any slack in the locking
lugs is therefore in front of the barrel lugs. Upon firing the barrel is
driven forward by the slug being forced through the bore, while the slide
is driven back by the recoil of the cartridge case. This rapidly takes up
whatever slack there is between the barrel lugs and the slide lugs. With
excess slack a momentum will build up and batter the locking lugs, making
things worse by developing more slack.
I am not addressing any of the other issues involved in
putting a tight accurate gun together, this is only about barrel fitting.
Grouping is spoiled when the barrel is allowed to move while the slug is
being driven down the bore. Particularly bad for grouping is rotary
movement because obviously the barrel will not rotate on the bore axis
with suspension at three points, the bushing, the hood and the underlug.
Accurizing is about eliminating variables. I try to put a
match grade gun together with minimum clearances, so that when it is shot a
few times and settles in, it will retain the accuracy for a very long time.